The invention relates to a method for controlling a hydraulic system of a working machine.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders. Although the invention will be described hereinafter with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as articulated haulers, dump trucks, graders, excavators or other construction equipment.
A working machine is provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load.
For example, a wheel loader has a load arm unit for raising and lowering an implement, such as a bucket. The load arm unit comprises a number of hydraulic cylinders for movement of a load arm and the implement attached to the load arm. A pair of hydraulic cylinders is arranged for lifting the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition to the hydraulic cylinders, the hydraulic system of the wheel loader comprises at least one pump for providing hydraulic fluid to the hydraulic cylinders of the load arm unit.
The hydraulic system of a wheel loader is usually a so called load sensing system (LS system). This means that the pump which provides the hydraulic system with hydraulic fluid receives a signal representing the current load pressure of a hydraulic cylinder in operation. The pump is controlled to provide a pressure which somewhat exceeds the load pressure of the hydraulic cylinder. Hereby a flow of hydraulic fluid to the current hydraulic cylinder is created.
In such a LS system energy is lost when one and the same pump is used for providing hydraulic fluid to several working functions. The working functions often require different pressures, which in turn implies that the pump has to be controlled to provide the highest pressure required by any working function. If two working functions are used at the same time and these working functions have different pressure demands, the pressure has to be reduced for the working function which requires the lowest pressure. By using a valve the pressure can be reduced to the desired pressure. The pressure drop over the valve results in heat energy loss.
An example of a wheel loader operation which involves loss of energy is when the wheel loader is forced into a pile of material in order to fill the bucket and take out material from the pile. During this moment the lift operation of the load arm is often stalled due to overload. The pressure in the hydraulic cylinders for lifting the load arm can be higher than the maximal pressure provided by the pump due to the fact that the propulsion force of the wheel loader retract the hydraulic cylinders. At the same time, the bucket is tilted in order to break off material from the pile and the tilt function is operated at a pressure which is lower than the pressure requested by the lifting function. The flow of hydraulic fluid to the tilt function will result in loss of energy since the pressure of the hydraulic fluid which is provided by the pump and flows to the tilt function has to be reduced from the maximal pump pressure to the pressure level required for the tilt function.
It is desirable to provide a method defined by way of introduction, by which method the loss of energy can be reduced in a hydraulic system of a working machine.
By the provision of a method where the pressure request from the first actuator is discriminated provided that the first actuator is stalled due to overload or geometrical limitations, the pump pressure can be adapted to another actuator of the hydraulic system, which actuator requires a flow of hydraulic fluid and a lower pump pressure, instead of keeping the pump pressure at the maximal level. This implies that the pump pressure does not need to be reduced by means of a valve, and since the heat energy loss is proportional to the pressure drop over a valve multiplied with the flow through the valve the loss of energy can be eliminated or at least reduced.
Furthermore, if no actuator in addition to the stalled actuator is present and in operation (i.e. requests a flow); the hydraulic machine can be controlled to provide a predetermined idle pump pressure which is lower than the maximal pump pressure. Since a hydraulic system always has some leakages a maximal pump pressure will in addition to unnecessary load and wear lead to energy losses when the hydraulic machine is controlled to maintain the maximal pump pressure also in the case where no actuator is in operation.
The method can of course be applied for any number of actuators. For example, should both the first and second actuators having the highest and second highest pressures be stalled, the pump pressure is preferably based on the load pressure of the actuator having the highest pressure of the remaining actuators.
Further advantages and advantageous features of the invention are disclosed in the following description.